Recently, as digital signal processing technology makes progress, coding apparatus for still pictures in digital still cameras, photo videotex and so on and coding apparatus for moving pictures utilized in video telephones, videoconferencing system, video CD system, DVD system, digital VTR(image processing system) and so on have been developed. For instance, coding apparatus utilizing orthogonal transform described in "Multi-dimensional Signal Processing for TV Pictures" written by Takahiko FUKINUKE (From page 213 to page 291, Chapter VII:High-efficient Coding, published Nov. 15, 1988 by Nikkan Kogyo Papers) and coding apparatus utilizing motion compensated interframe prediction are known.
In these apparatus, a digital picture discreted in M pixel in horizontal direction and N line in vertical direction are divided in blocks having m pixels in horizontal direction and n lines in vertical direction, and, in each block, high-efficient coding by motion compensated prediction processing, orthogonal transform processing and quantization processing orthogonal transform coefficient is carried out.
A conventional picture coding apparatus and a conventional picture decoding apparatus utilizing discrete cosine transform(abbreviated as DCT in the following) are explained below by using FIG. 9.
In FIG. 9A, 1 is an input terminal to which digital picture signals is input, 2 is an output terminal for outputting coded input picture signals, 11 is DCT processing section for processing two-dimensional DCT to each block of digital picture divided in blocks, 12 is quantizing section for quantizing DCT coefficients obtained by DCT processing section 11 and 13 is DCT coefficient coding section for transgenerating quantized DCT coefficients into code word.
And, in FIG. 9B, 3 is an input terminal to which code word is input, 4 is an output terminal for outputting decoded digital pictures, 22 is DCT coefficient decoding section for reproducing quantized DCT coefficients from code word, 23 dequantizing section to obtain non-quantized DCT coefficients from quantized DCT coefficients and 24 is inverse two-dimensional DCT processing section for reproducing a picture from DCT coefficients obtained by dequantizing section 23.
Each operation of the picture coding apparatus and the picture decoding apparatus constituted as described above is explained.
In the picture coding apparatus, an input picture is transformed into digital signals by an analog-digital converting circuit which is not shown and the input picture transformed into digital signals is divided in blocks in m pixels in horizontal direction and n lines in vertical directions. Each block of the input picture divided in block unit is input from input terminal 1 as digital picture signal 101. DCT processing section 11 acquires digital signal 101 comprised of one block as one unit and processes two-dimensional DCT to output DCT coefficient 102. Quantizing section 12 quantizes DCT coefficient 102 received from DCT processing section 11 and outputs DCT coefficient 103. Coding section 13 outputs code word 104 corresponding to quantized DCT coefficient 103 received from quantizing section 12.
In the picture decoding apparatus, code word 104 provided from input terminal 3 is transformed into quantized DCT coefficient 103 by DCT coefficient decoding section 22. Dequantizing section 23 transforms quantized DCT coefficient 103 input from DCT coefficient decoding section 22 into DCT coefficient 108.
Inverse two-dimensional DCT processing section 24 processes inverse two-dimensional DCT to DCT coefficient 108 to obtain reproduced picture 109.
By the way, block dividing digital pictures then coding a digital picture by quantizing orthogonal transform and orthogonal transform coefficient in block unit as described above results in the difference between the signal value before coding processing and the decoded signal value in a block, which is caused by error in DCT/inverse DCT and quantizing error. That brings signal non-continuity in neighbor blocks, then noise called block distortion generates.
As measures to prevent this noise, a method for giving mask processing to orthogonal transform coefficient in picture coding apparatus (for instance, Separate volume 1-202 of Collections of National Conference Lecture Theses in Information/System Division of Electronic Communication Society in 1985) and a method for restoring a decode picture by using a post filter in a picture decoding apparatus (for instance, Separate volume 7 D-307 of Collections of Spring National Conference Lecture Theses in Electronic Information Communication Society in 1990) are proposed.
However, these methods can not decide which makes the non-continuity in block boundary, error caused by coding-decoding processing or edge component which the original input picture have, then it is impossible to completely prevent noise generation and to collect the reproduced picture.